Recently, the development of novel water resource supply sources has come to the fore as an urgent issue due to severe levels of water contamination and water shortages worldwide. Research into the contamination of water is aimed at processing water for domestic, commercial and industrial uses, various types of domestic sewage, industrial waste water and the like. Water-treatment processes, using separation membranes having advantages in terms of energy savings, for example, have come to prominence. Further, the enforcement of environmental regulations increasing in recent times, may allow for the advanced activation of separation membrane technologies. In meeting the requirements of such environmental regulations, traditional water-treatment processes may be insufficient; however, since separation membrane technologies may ensure superior processing efficiency and stable treatment processes, they are expected to become leading technologies in the field of water treatment going into the future.
Liquid separation methods may be classified as a micro-filtration method, an ultra-filtration method, a nano-filtration method, a reverse osmosis method, a stannizing method, an active transportation method, an electrodialysis method and the like. Among these methods, the reverse osmosis method may be a desalinization process using a semipermeable membrane enabling water to penetrate therethrough while not allowing salts to penetrate therethrough. When high pressure water in which salts are dissolved is introduced to one surface of the semipermeable membrane, de-ionized water from which salts are removed may be discharged through the other surface of the semipermeable membrane at low pressure.
Lately, worldwide, about a billion gallons of water are subjected to a desalinization process through a reverse osmosis method every day. Since the first desalinization process using reverse osmosis was disclosed in the 1930s, a considerable amount of research into semipermeable membrane materials has been conducted in the field thereof. Here, research into asymmetric cellulose membranes and polyamide composite membranes has become prominent in terms of commercial viability. Since cellulose membranes developed in the early stage of reverse osmosis membrane technological development may be disadvantageous in that they have relatively narrow operable Ph ranges, are easily deformed at high temperatures, require high operational costs due to the use of high pressure and are vulnerable to microbes, they have been rarely used in recent times.
Meanwhile, a polyamide composite membrane may be prepared by forming a polysulfone layer on a non-woven fabric to form a microporous support, dipping the microporous support in an aqueous m-phenylene diamine (mPD) solution to form an mPD layer, and dipping or coating the mPD layer in or with an organic trimesoyl chloride (TMC) solvent to allow the mPD layer to be brought into contact with the TMC so as to be interfacially polymerized to thereby form a polyamide layer. A non-polar solution and a polar solution may come into contact with each other, whereby the polymerization may only be generated in the interface to form a polyamide layer having a significantly small thickness. Since polyamide composite membranes have high levels of stability with respect to pH variations, are operatable under low degrees of pressure, and have superior salt rejection rates, as compared to existing asymmetric cellulose membranes, they are now mainly provided as water-treatment separation membranes.
However, the polyamide composite membranes have defects in that the replacement period of the membranes is relatively short, because a reduction ratio in chlorine resistance over time is high. Thus, in order to lower the reduction ratio in chlorine resistance of water-treatment separation membranes, a method of increasing a specific surface area of an active layer thereof has been suggested. Specifically, a technology of forming an active layer such that a specific surface area of a skin layer of the water-treatment separation membrane is large, and subsequently, dipping the active layer into an acid solution to form a surface of the skin layer to be uneven or to form wrinkles in the layer has been disclosed. Also, a method of fabricating a reverse osmosis composite membrane and subsequently, performing post-processing on the membrane with strong acid to thereby increase surface roughness has been disclosed.
However, in a case in which a separation membrane provided with an active layer is dipped in an acid solution, a surface of the separation membrane may have a negative charge, such that contaminant materials having positive charges may be attached to the separation membrane to cause a decrease in transmittance of the separation membrane. Accordingly, a separate post-processing process of coating the surface of the separation membrane with electrically neutral polymers needs to be undertaken.
Further, polyamide separation membranes according to the related art are significantly sensitive to oxidizing materials, and the oxidizing materials may remarkably deteriorate functions of the membranes with only a low concentration and may rapidly reduce the salt rejection rate. As the majority of water resources worldwide are contaminated, the water resources may be acid-treated wastewater and water treated by a separation membrane may contain a great quantity of disinfectant at all times. Consequently, the lifespan of the related art separation membrane used in processing such water may be short and it may be necessary to frequently replace the separation membrane. Thus, membrane efficiency may be decreased while costs required for treatment may be increased.